Variable and self-regulating permeate recycling in organophilic nanofiltration

ABSTRACT

The invention provides an improvement in terms of control and process technology for a method of continuous removal of a component from a liquid mixture using a membrane unit comprising at least one membrane stage. The improvement is that a portion of the overall permeate stream is recycled to the feed vessel and/or beyond the feed vessel but upstream of the conveying device, and the remainder of the overall permeate stream is removed, with the recycled permeate having a higher concentration of the component to be separated off than the removed permeate. The presently disclosed method can especially be used for separation of a homogeneously dissolved catalyst from a liquid reaction mixture.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a 35 U.S.C. § 119 patent application which claimsthe benefit of European Application No. 20216295.4 filed Dec. 22, 2020,which is incorporated herein by reference in its entirety.

FIELD

The present invention relates to an improvement in terms of control andprocess technology for a method of continuous removal of a componentfrom a liquid mixture using a membrane unit comprising at least onemembrane stage. The improvement is that a portion of the overallpermeate stream is recycled to the feed vessel and/or beyond the feedvessel but upstream of the conveying device, and the remainder of theoverall permeate stream is removed, with the recycled permeate having ahigher concentration of the component to be separated off than theremoved permeate. The presently disclosed method can especially be usedfor separation of a homogeneously dissolved catalyst from a liquidreaction mixture.

BACKGROUND

Membrane separation methods, for example for the separation ofhomogeneously dissolved catalysts from the reaction mixture, are knownin principle in the prior art. Reference is made here by way of exampleto WO 2014/131623 A1.

For membrane separation methods of this kind, improvements in controltechnology have already been described. International application WO2014/183952 A1 discloses, for example, a membrane separation method forseparation of a homogeneously dissolved catalyst from a reactionmixture, in which two parameters, the retentate volume flow rate of themembrane separation unit and the retention of the membrane separationunit, are controlled to remain constant in order to compensate forfluctuating operating conditions, especially fluctuations in the volumeflow rate of the reaction mixture coming from the reaction zone.

In order to keep the aforementioned controlled variables constant, WO2014/183952 A1 proposes a flow resistor for adjusting the retentatevolume flow rate, or keeping it constant, and closed-loop control of thetemperature and/or of the pressure in the overflow circuit for adjustingthe retention, or keeping it constant.

A problem with the method proposed therein is that it is not productivefor all methods to keep the retention of the membrane separation unitconstant; instead—taking account of the plant throughput and themembrane area already installed in the membrane separation unit—what isrequired is optimization, frequently maximization, of the retention ofthe membrane separation unit during the method. Closed-loop control bymeans of a flow resistor as described in WO 2014/183952 A1 may possiblyhave the effect that the membrane area already installed is operated notat the optimal transmembrane pressure but below it. The method describedtherein can additionally have the effect that, for a given totalthroughput of the plant and the resulting permeate stream removedtherefrom (portion of the permeate stream conducted out of the membranestage or the membrane separation unit on the permeate side), the totalpermeate stream and the resulting recycled permeate stream on the basisof a simple mass balance taking account of the permeate stream removed(portion of the total permeate stream which is recycled into themembrane separation unit) is smaller than possible and/or would bedesirable.

A further disadvantage of the method described in WO 2014/183952 A1 isthat it envisages both a vessel for the permeate and a pump in order toobtain a closed-loop control system which is constant on the permeateside. The provision of a pump that pumps permeate from the permeatevessel into the overflow circuit or the feed vessel, however, isassociated with an elevated plant inventory and costs, for example forprocurement or for operation, maintenance and repair, which canadditionally also result in a plant shutdown and hence productionshutdown.

In addition, the installation of a vessel generally has the effect thatthe portion of the membrane separation stage on the permeate side is notfilled hydraulically, which can possibly lead to elevated safety risks,especially when toxic fluids are used. Moreover, a vessel on thepermeate side in addition to the feed vessel gives rise to thepossibility that the closed-loop control of the two vessel states maylead to opposite fluctuations.

SUMMARY

The problem addressed by the present invention was therefore that ofproviding a less expensive method of continuously separating a componentfrom a mixture, preferably for continuous separation of a homogeneouscatalyst from a reaction mixture. A further problem addressed by thepresent invention was that of providing self-regulating permeaterecycling in the method of continuous separation of a component from amixture, preferably of continuous separation of a homogeneous catalystfrom the reaction mixture, in which the membrane area installed can beutilized optimally in different load states, for example formaximization of the yield.

The underlying problem was solved by the method set forth herein.Preferred configurations and embodiments are specified herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic of a membrane unit comprising a membrane stagewith two membrane modules.

FIG. 2 shows a schematic of a membrane unit comprising two membranestages.

DETAILED DESCRIPTION

The method according to the invention is a method of continuouslyseparating a component from a liquid mixture using a membrane unit whichcomprises at least one membrane stage and is fed with the mixture asfeed, wherein a membrane stage consists at least of a conveying device,at least two series-connected membrane modules, and a feed vesselupstream of the conveying device,

wherein the method comprises the following steps in which:

the mixture from the feed vessel is guided by means of the conveyingdevice as feed to the first of the at least two membrane modules, whichdepletes the component to be separated off, based in each case on themixture guided to the first membrane module, in the resulting permeatestream from the first membrane module and enriches it in the resultingretentate stream from the first membrane module,

the retentate from the first membrane module is guided to the secondmembrane module, which depletes the component to be separated off, basedin each case on the retentate from the first membrane module, in theresulting permeate stream from the second membrane module and enrichesit in the resulting retentate stream from the second membrane module,

characterized in that

the at least two membrane modules are joined to one another on thepermeate side by such a means that the at least two permeate streams areeach guided to a common pipeline, which gives rise to an overallpermeate stream in the pipeline, and

the at least two membrane modules are connected to the common pipelineon the permeate side in opposite directions such that the portion of theoverall permeate stream, the recycled permeate, is recycled at leastpartly to the feed vessel and/or beyond the feed vessel but upstream ofthe conveying device, while the remainder of the overall permeatestream, the removed permeate, is guided at least partly out of themembrane stage, with the recycled permeate having a higher concentrationof the component to be separated off than the removed permeate.

In the context of the present invention, what is meant by the term“component to be separated off” is the component intended to permeatethrough the membrane to a lesser degree in terms of its proportionand/or according to permeability, i.e. the component that is retained bythe membrane. The component to be separated off thus has positiveretention for the particular membrane modules in question. The componentto be separated off may additionally be a single specific chemicalsubstance or else a group of chemical substances that can be consideredin a common context for process technology purposes.

The method according to the invention may also comprise more than twomembrane modules. If there are three or more membrane modules, it ispreferable that the retentate from the previous membrane module isguided to the subsequent membrane module (not applicable to lastmembrane module), as a result of which a component to be separated off,based in each case on the retentate from the previous membrane module,is depleted in the resulting permeate stream from the subsequentmembrane module and is enriched in the resulting retentate stream fromthe subsequent membrane module.

“Membrane unit” in the context of the present invention relates to theentire membrane separation unit supplied with the liquid mixturecontaining the component to be separated off. A membrane unit consistsof at least one membrane stage. Downstream further processing orpurification steps and any storage of the permeate removed in a vesselare therefore not part of the membrane unit by definition.

The term “membrane stage” means at least a portion of the membrane unitand includes at least one conveying device, for example a pump, and atleast two membrane modules. If there is just a single membrane stage,the terms “membrane unit” and “membrane stage” should be understoodsynonymously. The membrane stage has a feed vessel connected upstream ofthe conveying device, into which the liquid mixture, for example theoutput from a homogeneously catalyzed reaction, is introduced, and fromwhich it is guided to the one or more membrane elements. The permeaterecycled can likewise be guided into the feed vessel. Additionally oralternatively, it is possible that the permeate recycled is guided notinto the feed vessel, but rather beyond the feed vessel but upstream ofthe conveying device, i.e. at a point in the conduit between feed vesseland conveying device.

The term “membrane module” utilized for description of the presentinvention relates to a subunit, in terms of plant technology, of themembrane stage. A membrane module is accordingly an interconnection ofone or more membrane element(s). Membrane modules may be configuredeither as a membrane loop or as a membrane rack. A membrane loop isunderstood to mean a subunit in which there is at least one membraneelement and at least one conveying device by which a moving overflowcirculation is generated. The term “membrane rack”, by contrast, means asubunit having the feature that there is at least one membrane elementbut no conveying device, and accordingly no moving overflow circulationis generated either.

The term “membrane element” in the context of the present inventionmeans the membrane or the structure or the apparatus containing themembrane, and where the desired separation of matter actually takesplace, i.e. the component is separated from the mixture or thehomogeneous catalyst from the reaction solution. These may be, forexample, spiral-wound elements as used in many applications inindustrial membrane separation.

The basis of the invention is that only the overall permeate stream fromthe last membrane stage is divided into the permeate recycled and thepermeate removed. Should there be only one membrane stage, this singlemembrane stage is also the last membrane stage. If there are two or moremembrane stages, no division takes place in the first membrane stage(s);instead, the entire permeate stream is guided to the next membrane stagein its entirety. The division is then effected, as described above,exclusively in the last membrane stage. The term “division” in thecontext of the present invention relates exclusively to the amount ormass flow rate of the overall permeate and means explicitly noadditional separation step in which the further components are removedfrom the overall permeate, i.e., for example, no distillation, noextraction, no crystallization, no adsorption and no further membraneseparation steps.

One advantage of the method according to the invention is that a portionof the overall permeate stream from the last membrane stage, i.e. thepermeate recycled, does not leave the membrane stage and/or does notleave the membrane unit to only then be recycled via a permeate vessel,for example, but rather remains within the at least one membrane stageand/or the membrane unit and thence is recycled to the feed vesseland/or beyond the feed vessel but upstream of the conveying device. Thismakes it possible to dispense with a downstream pump that pumps thepermeate back into the overflow circuit or back into the feed vessel,since the permeate recycling is self-regulating according to the load.The recycling of the recycled permeate to the feed vessel and/or beyondthe feed vessel but upstream of the conveying device is thus effectednot by means of a conveying device but rather by hydraulic means, i.e.by means of a pressure differential existing between the permeate sideof the membrane unit and the suction side of the conveying device or thefeed vessel. In other words, there is no further conveying device, inparticular no pump, between the permeate side and the feed vessel.

The portion of the overall permeate stream which is removed from themembrane stage or the membrane unit, i.e. the permeate removed, can beguided to a downstream process step. The term “process step” in thecontext of the present invention may be understood to mean anydownstream process, for example further processing or purificationprocess steps or combinations thereof. This especially includes a(further) reaction of the component removed, purification by knownmethods such as distillation, evaporation or the like. Downstreamdispensing or transport process steps are also possible. Before thedownstream process step, prior storage in a suitable vessel is alsopossible, for example in a permeate vessel. If there is a permeatevessel, the division of the overall permeate according to the inventionis effected upstream of the permeate vessel, such that only the permeateremoved gets into the permeate vessel. It will be apparent that multipleprocess steps, optionally via prior storage, may be effected insuccession, for example a purification followed by a conversion of thepurified component and additional purification of the reaction product.

According to the invention, the at least two membrane modules areconnected to a common pipeline on the permeate side in oppositedirections such that the portion of the overall permeate that comes atleast predominantly from the last membrane module on the retentate side(the recycled permeate) is recycled at least partly to the feed vesseland/or beyond the feed vessel but upstream of the conveying device,while the remainder of the overall permeate that comes at leastpredominantly from the first membrane module on the retentate side (theremoved permeate) is guided at least partly out of the membrane stage.The permeate from all membrane modules present in such a membrane stageis thus guided to a common pipeline and separated therein. In the commonpipeline, between the connections of the first and last membrane modulesto the pipeline, there are preferably no controlling actuators, forexample valves or separating units. However, this does not rule out thepresence of non-regulating actuators, i.e. actuators that are presentbut do not intervene in the process, for example a constantly andcompletely open valve. What is achieved by the concept of the inventionin a simple manner, in terms of plant technology, is that the recycledpermeate has a higher concentration of the component to be separated offthan the permeate removed. These plant- and control-related functions,in the unfavorable case that there is no feed to the feed vessel from anupstream process step, have the effect that complete recycling of thepermeate from all modules to the feed vessel or into the membrane unittakes place.

The present method ensures that the different permeates from the atleast two membrane modules do not mix completely, meaning that a mixturehaving the same concentration of the component to be separated off isformed in the common pipeline. What is ultimately to be achieved is thatthe recycled permeate has a higher concentration of the component to beseparated off than the permeate removed, which would not be achieved inthe case of complete mixing. This can also be illustrated by FIG. 1: forinstance, in the course of the process of the invention, the result mustbe that the concentration of the component to be separated off in therecycled permeate is greater than in the permeate removed. Based on theconcentration of the component to be separated off, it is accordinglyalways the case that F-11>F-12. A preferred additional result is thatthe concentration of the component to be separated off in the recycledpermeate is greater than in the common pipeline between the connectionsof the first and last membrane modules, with the concentration of thecomponent to be separated off being greater therein than in the permeateremoved. Based on the concentration of the component to be separatedoff, it is accordingly the case that F-11>F-10>F-12. The prevention ofcomplete mixing can be achieved in various ways by plant technology. Apreferred option according to the present invention is thecharacteristics of the common pipeline (for example length, diameter,etc.), which are adjusted so as to avoid complete mixing of the overallpermeate.

In order to control the exit flow rates of the permeate recycled and thepermeate removed, there may be at least one adjustable flow resistor onthe permeate side, by means of which the mass flow rate of the permeateis controlled. A flow resistor in the context of the present inventionis an actuator with which the mass flow rate of a stream can becontrolled, for example a valve. Preference is further given to anembodiment in which there are at least two adjustable flow resistors,preferably exactly two adjustable flow resistors, on the permeate side,which means that the mass flow rate of the permeate removed and thepermeate pressure, especially of the common pipeline, can be adjusted.The at least two adjustable flow resistors are especially valves. Thevalves are preferably present not between the connections of the firstand last membrane modules to the pipeline, but are beyond saidconnections to the pipeline in flow direction of the permeate recycledor removed.

With regard to the membrane separation method according to theinvention, there is a stable external mass balance, meaning that themass flow rate of the feed supply to the membrane unit corresponds tothe mass flow rates of the permeate streams and/or retentate streamsconducted out of the membrane units. It is possible to make use of thiscircumstance. In a preferred embodiment of the present invention, themass flow rate of one of the three streams selected from feed to themembrane unit, the permeate removed, and the retentate from the membraneunit is defined by a preceding or downstream process step, and onefurther stream of the three is controlled toward a target value, forexample a constant retentate flow rate, a constant ratio of feed toretentate, etc. On account of the external mass balance, this alsoresults in the third of the three streams mentioned. The scale and hencethe absolute mass flow rates are adjustable largely as desired withreference to the membrane module size and number of membrane modules. Ina preferred embodiment, the ratio of the retentate mass flow rate to themass flow rate is 1% to 99%, preferably 10% to 90% and more preferably15% to 80%. In a further preferred embodiment, the ratio of the massflow rate of the permeate removed to the overall permeate mass flow rateis 1% to 99%, preferably 30% to 98%, more preferably 60% to 97%.“Process step” in the context of the present invention is a plant orprocess unit, for example an upstream or downstream chemical reaction inwhich the permeate/retentate is used, a further separation step, forexample a thermal separation such as thin-film evaporation ordistillation, or logistics, i.e. especially an upstream or downstreamtank farm or a dispensing operation. Upstream process steps areespecially continuously performed process steps by means of which aliquid mixture is provided continuously for the present membraneseparation process. These are preferably continuous performed chemicalreactions, for example hydroformylation or alkoxycarbonylation, whichare elucidated in detail hereinafter.

In addition, more particularly, there is also a stable internal massbalance (mass flow rate of the overall permeate, i.e. the sum total ofall permeate streams from the membrane modules, corresponding to the sumtotal of the mass flow rates of the permeate recycled and the permeateremoved). According to the present invention, the internal mass balanceis preferably largely independent of the external mass balance, meaningthat the outer mass balance is in principle merely the lower limit forthe internal mass balance. It is therefore preferable in accordance withthe invention that the mass flow rate of the permeate recycled canfluctuate and is established depending, preferably directly depending,i.e. without any permeate vessel in between, on the mass flow rate ofthe permeate removed. This also means that the mass flow rate of theoverall permeate, disregarding technical limitations (resulting frompumps, flow resistors, membrane area, etc.), can be controlledindependently of the abovementioned external mass balance, provided thatthe mass flow rate of the overall permeate is greater than the mass flowrate of the permeate removed.

The mass flow rate of the overall permeate depends here on variousparameters, for example the temperature (of the membrane modules) or ofthe concentration of the components in the mixture. In a preferredembodiment of the present invention, therefore, the pressure on theretentate side and/or the pressure on the permeate side or the resultingtransmembrane pressure (TMP=pressure differential between permeate sideand retentate side) and optionally the membrane module temperature arecontrolled in order to optimize the amount of the overall permeatestream or to obtain a desired amount of the overall permeate stream.

The membrane separation method according to the invention can beadjusted in various ways in terms of control technology, depending onthe respective controlled variable, an actuator present for influencingthe controlled variable, and control priority. There are variouscontrolled variables for the present method, such as the fill level ofthe feed vessel, the pressure on the retentate side and on the permeateside, the differential of which results in the transmembrane pressure(TMP), and the mass flow rates of retentate and permeate that can beinfluenced by various actuators, for example the conveying device or oneor more adjustable flow resistors.

The membrane separation method according to the invention, in apreferred embodiment, is adjusted in terms of control technology in sucha way that the mass flow rate of the retentate and the TMP are keptconstant. These two parameters consequently have the highest controlpriorities, where the exact sequence of the control priorities can befixed as desired, i.e. the mass flow rate of the retentate may have thehighest control priority and the TMP the second highest controlpriority, or vice versa.

The mass flow rate of the feed to the (first) membrane module in themethod according to the invention may be established in a manner knownto the person skilled in the art, for example via the conveying deviceused for the (first) membrane stage. The exact embodiment forestablishment of the mass flow rate of the feed to the (first) membranestage is variable and usually depends on technical constraints, forexample the type of pump chosen, the conveying rate and the conveyingpressure. It would be possible to implement control of the feed massflow rate, for example, with a pump controlled directly by means ofspeed, for example a gear pump, a piston pump, a piston membrane pump oroptionally a multistage centrifugal pump. Another option for control ofthe mass flow rate may be that of using a centrifugal pump and anadjustable flow resistor, such as a (control) valve. A further optionwould be the use of a pump, for example a gear pump, a piston pump, apiston membrane pump or a centrifugal pump, in combination with anadjustable return flow conduit, for example from the pressure site tothe suction side of the pump.

The pressure on the retentate side (retentate pressure) can becontrolled by means of the conveying device and/or optionally a furtheractuator, for example a supply pressure regulator. The retentatepressure in the method according to the invention may be 1 to 100 bar,preferably 10 to 80 bar and more preferably 30 to 60 bar. The retentatepressure is greater than the pressure on the permeate side (permeatepressure). The transmembrane pressure formed by the differential ofretentate pressure and permeate pressure may be 1 to 90 bar, preferably10 to 80 and more preferably 30 to 60 bar.

The permeate pressure may then be 0 to 50 bar, preferably 0 to 10 barand more preferably 1 to 5 bar. The permeate pressure of all membranemodules present, in a preferred embodiment, is similar (with not morethan 10% mutual variance) or the same, which can further preferably beestablished via a supply pressure regulator on the permeate side. Thisis because the pressures of the individual membrane modules arepreferably not controlled independently of the total pressure on thepermeate side, for example the common pipeline.

The membrane unit, or the individual membrane stages, preferablycomprise(s) a closed-loop control system for the pressure on theretentate side, comprising at least the conveying device and amanometer, where the retentate pressure can be adjusted depending on themanometer. The retentate pressure here may be controlled by adjusting,for example, the conveying volume of the conveying device and optionallyby means of a further actuator, for example a supply pressure regulator,depending on the retentate pressure measured (by the manometer),wherein—based on a target value fixed beforehand for the retentatepressure—the conveying volume of the conveying device is reduced whenthe retentate pressure is elevated and/or rising and the conveyingvolume of the conveying device is increased when the retentate pressureis reduced and/or falling.

The retentate pressure may alternatively be controlled via a combinationof a manometer and an adjustable flow resistor, especially a valve onthe retentate side. The retentate pressure can be controlled here, forexample, via the valve setting, depending on the retentate pressuremeasured (by the manometer), wherein—based on a target value fixedbeforehand for the retentate pressure—the valve is opened further whenthe retentate pressure is elevated and/or rising and closed further whenthe retentate pressure is reduced and/or falling.

The mass flow rate of the retentate (retentate mass flow rate) in themethod according to the invention is preferably controlled by means of amass flow regulator on the retentate side, comprising at least one massflow meter and an adjustable flow resistor, preferably a valve. Theretentate mass flow rate can be controlled here by adjusting the massflow regulator depending on the retentate mass flow rate measured,wherein—based on a target value fixed beforehand for the retentate massflow rate—the valve of the mass flow regulator is closed further whenthe retentate mass flow rate is elevated and/or rising and the valve ofthe mass flow regulator is opened further when the retentate mass flowrate is reduced and/or falling. The retentate pressure can then bechosen freely within the scope of the minimum necessary and maximumpossible loading of the membrane stage(s).

In another embodiment, it is alternatively possible that the retentatemass flow rate is controlled by means of a combination of mass flowmeter and the conveying device. The retentate mass flow rate can becontrolled here, for example, by adjusting the conveying volume of theconveying device depending on the retentate mass flow rate measured,wherein—based on a target value fixed beforehand for the retentate massflow rate—the conveying volume of the conveying device is reduced whenthe retentate mass flow rate is elevated and/or rising and the conveyingvolume of the conveying device is increased when the retentate mass flowrate is reduced and/or falling.

The sensors and actuators for the closed-loop control of mass flow rateon the retentate side, comprising at least the conveying device or anadjustable flow resistor and a mass flow meter, and the aforementionedclosed-loop control of pressure on the retentate side, comprising atleast the conveying device or an adjustable flow resistor and amanometer, may be connected to one another in any desired manner forcontrol technology purposes, in order to control the two controlledvariables of retentate mass flow rate and retentate pressure. The mutualpriority of the two controlled variables of retentate mass flow rate andretentate pressure can be chosen as desired in the method according tothe invention. The priority of the two controlled variables of retentatemass flow rate and retentate pressure (and hence the TMP) is preferablyhigher than that of all other controlled variables in the membraneseparation stage, i.e. has the fastest response characteristics.

The mass flow rate of the permeate removed (from the last membranestage), depending directly or indirectly on the fill level of the feedvessel, is subject to preferably continuous closed-loop control, afeature of which is that—based on a target value fixed beforehand forthe fill level of the feed vessel, where the target value is preferably20% to 80%, more preferably 30% to 70%, of the maximum possible filllevel—the mass flow rate of the permeate removed increases with risingfill level of the feed vessel and the mass flow rate of the permeateremoved decreases with falling fill level of the feed vessel. In thistype of closed-loop control, the feed vessel is not completely filledsince the fill level is otherwise at or above the upper edge of itsmeasurement range and hence is unknown, as a result of which noclosed-loop control to a target value is possible. The mass flow rate ofthe permeate removed is especially adjusted via at least one adjustableflow resistor. In the case of continuous closed-loop control, thispreferably keeps the fill level of the feed vessel constant. The effectof the closed-loop control principle according to the fill level of thefeed vessel would be, for example, that, on attainment of a low level inthe feed vessel, the mass flow rate of the permeate removed is reducedever further and possibly no further permeate is removed.

The temperature of the three streams of feed, retentate and permeate mayvary within a wide range. The temperature of each of the three streamsof feed, retentate and permeate is independently preferably −30° C. to150° C., more preferably 0° C. to 100° C. and most preferably 20° C. to80° C.

A contribution to the achievement of the aforementioned modes ofexecution of the method according to the invention in terms of controltechnology is also made by the construction of the membrane unit, whichis to be described in more detail hereinafter.

The membrane unit which is used in the method according to the inventionfor separating the component from the mixture comprises at least onemembrane stage. The membrane unit may alternatively comprise multiplemembrane stages connected in series to one another. In this case, theoverall permeate stream is divided only in the last membrane stage.

A membrane stage of the membrane unit for the method according to theinvention, according to the above definition, comprises a conveyingdevice. The conveying device with which the mixture is guided as feed tothe at least two series-connected membrane modules is preferablyadjustable with regard to its conveying volume. The pressure of the feedto the at least two membrane modules may be 1 to 100 bar, preferably 10to 80 bar and more preferably 30 to 60 bar. Suitable conveying devicesare, for example, pumps known to the person skilled in the art, such ascentrifugal pumps, piston pumps, piston membrane pumps, rotary pistonpumps or gear pumps.

A membrane stage of the membrane unit according to the invention furthercomprises at least two series-connected membrane modules. There istheoretically no upper limit to the number of membrane modules; instead,it depends on the general process parameters and the desired membranearea. In a preferred embodiment, the membrane stage comprises more thantwo membrane modules that are further preferably connected in series toone another. The mixture arriving at the membrane unit is guided to the(first) membrane stage, where it is guided to the one or more membranemodules by means of the conveying device as feed. The separation intopermeate and retentate is effected within the membrane stage, with apermeate stream being withdrawn from each membrane module present.

In the at least two membrane modules present here, accordingly, a numberof permeate streams corresponding to the number of membrane modules isgenerated. The membrane modules are connected here to the commonpipeline on the permeate side in opposite directions as mentioned above.By contrast, only one retentate stream is obtained, since the retentatefrom the first membrane module is guided to the next membrane module, afurther permeate is separated from the retentate, and the retentate fromthe second membrane module is then guided to the next membrane moduleor, if there are only two membrane modules, it is conducted out of themembrane stage and/or the membrane unit.

A membrane stage in the method according to the invention comprises avessel upstream of the conveying device, from which the feed is guidedto the at least two membrane modules as feed by means of the conveyingdevice. If there is just one membrane stage, it is possible for both thefeed to the membrane stage and the recycled permeate from the membranestage to be introduced into the feed vessel and collected therein beforethey are guided by means of the membrane device as feed to the at leasttwo membrane modules. If there is more than one membrane stage, the feedto the first membrane stage and a retentate from one of the subsequentstages can be collected in the feed vessel for the first stage, whilethe permeate from the respective preceding membrane stage and either theretentate from the subsequent stage or, in the last membrane stage,recycled permeate can be collected in the feed vessels for thesubsequent stage(s). The construction and specifications of such a feedvessel are known to the person skilled in the art.

The feed vessel preferably comprises a measurement unit for the filllevel. It is also possible in those variants that the respectiverecycled streams are guided not into the feed vessel but rather beyondthe feed vessel but upstream of the conveying device.

The membrane stage may further comprise sensors and/or actuators inorder to be able to fulfil the aforementioned, preferred control-relatedfunctions. These especially include measurement and/or control units forthe parameters such as temperature, pressure, mass flow rate or thelike. Corresponding measurement and control units are known to theperson skilled in the art.

A membrane module according to the present invention, of which there areat least two in the membrane stage, comprises one or more membraneelement(s). In principle, membrane modules may, as mentioned, beconfigured either as a membrane loop or as a membrane rack. The membranemodules present in accordance with the invention in the membranestage(s) are preferably membrane loops.

A membrane loop comprises one or more membrane element(s) and at leastone conveying device. A membrane loop preferably comprises just oneconveying device. This conveying device is not identical to theconveying device for the corresponding membrane stage; instead, in thatcase, the entire system has at least two conveying devices. Theconveying device of the membrane loop is generally responsible for thecirculation of the membrane loop, while the conveying device of themembrane stage is generally responsible for the pressurization of themembrane modules or of the membrane loop. The conveying device used maybe any suitable pump. Such pumps are known to the person skilled in theart. The pump used as conveying device within a membrane loop ispreferably a centrifugal pump. The conveying device generates a movingoverflow circulation. The overflow circulation can ideally improve masstransfer and hence the separation performance of the membrane. Theoverflow circulation can be adjusted here independently of theoverriding control concept and the external and internal mass balance.

A membrane loop may additionally also comprise measurement and/orcontrol units for parameters such as temperature, pressure differential(axial pressure drop), circulation rate or the like, for example aheater or cooler in order to adjust the temperature. Further measurementand control units are known to the person skilled in the art. In apreferred embodiment, the pressure in all membrane loops present in amembrane stage is similar (variance <10%) or identical. This at leastsimilar pressure can be established without the presence of a particularcontrol unit, but may also be established by means of a pressureregulator. The pressure regulator for the permeate pressure preferablyhas the slowest response characteristics compared to the otheractuators, i.e. the closed-loop control of mass flow on the permeateside, the closed-loop control of mass flow on the retentate side and theclosed-loop control of pressure on the retentate side.

By contrast with the above-described membrane loop, a membrane moduleconfigured as a membrane rack does not have a conveying device, butrather one or more membrane elements and optionally additionalmeasurement and control units.

The membrane element(s) present in the membrane module, preferably inthe membrane loop, are elements prefabricated for industrial use thatcontain the membrane and can be considered as a base unit that cannot bedivided any further in the membrane separation method according to theinvention. The membrane element(s) may be used as such in the membranemodule or be disposed in a pressure housing, for example a pressuretube. The pressure tube considered in isolation may contain one or moremembrane element(s), preferably up to five membrane elements. If themembrane element(s) are disposed in a pressure housing, preferably apressure tube, a membrane module may comprise multiple pressure tubes.The flow preferably passes in series on the feed or retentate sidethrough the membrane elements disposed in a pressure tube, and they arepreferably connected on the permeate side. Base units used as membraneelement may be spiral-wound elements known to the person skilled in theart. One or more spiral-wound elements may then be present in a pressurehousing, preferably a pressure tube.

Membranes used are preferably those having a separation-active layermade of a material selected from the group consisting of celluloseacetate, cellulose triacetate, cellulose nitrate, regenerated cellulose,polyimides, polyamides, polyetheretherketones, sulfonatedpolyetheretherketones, aromatic polyamides, polyamidoimides,polybenzimidazoles, polybenzimidazolones, polyacrylonitrile,polyarylethersulfones, polyesters, polycarbonates,polytetrafluorethylene, polyvinylidene fluoride, polypropylene,terminally or laterally organomodified siloxane, polydimethylsiloxane,silicones, silicone acrylates, polyphosphazenes, polyphenylsulfides,polybenzimidazoles, Nylon® (nylon-6,6), polysulfones, polyanilines,polypropylenes, polyurethanes, acrylonitrile/glycidyl methacrylate(PANGMA), polytrimethylsilylpropynes, polymethylpentynes,polyvinyltrimethylsilane, polyphenylene oxide, alpha-aluminium oxides,gamma-aluminium oxides, titanium oxides, silicon oxides, zirconiumoxides, ceramic membranes hydrophobized with silanes, as described in EP1 603 663 B1, polymers with intrinsic microporosity (PIM) such as PIM-1and others, as described, for example, in EP 0 781 166 B1, or mixturesthereof. The abovementioned substances may be in crosslinked form in theseparation-active layer through addition of auxiliaries, or in the formof what are called mixed matrix membranes with fillers, for examplecarbon nanotubes, metal-organic frameworks or hollow spheres, andparticles of inorganic oxides or inorganic fibers, for example ceramicfibers or glass fibers.

Particular preference is given to using membranes having, as aseparation-active layer, a polymer layer of terminally or laterallyorganomodified siloxane, polydimethylsiloxane, silicone acrylates orpolyimide, formed from polymers having intrinsic microporosity (PIM)such as PIM-1, or wherein the separation-active layer has been formed bymeans of a hydrophobized ceramic membrane. Very particular preference isgiven to using membranes formed from terminally or laterallyorganomodified siloxanes or polydimethylsiloxanes. Membranes of thiskind are commercially available.

As well as the abovementioned materials, the membranes may includefurther materials. More particularly, the membranes may include supportor carrier materials to which the separation-active layer has beenapplied. A selection of support materials is described by EP 0 781 166,to which reference is made explicitly.

In a particularly preferred embodiment, the method described is utilizedfor membrane separation of a homogeneous catalyst. The component to beseparated off is then the homogeneous catalyst, and the liquid mixtureis the reaction mixture obtained from a reaction stage.

A particularly preferred method according to the present invention istherefore a method of continuously separating a homogeneous catalystfrom a liquid reaction mixture using a membrane unit which comprises atleast one membrane stage and is fed with the reaction mixture containingthe homogeneous catalyst and coming from a reaction zone as feed,wherein a membrane stage consists at least of a conveying device, atleast two series-connected membrane modules, and a feed vessel upstreamof the conveying device,

wherein the method comprises the following steps in which:

the reaction mixture from the feed vessel is guided by means of theconveying device as feed to the first of the at least two membranemodules, which depletes the homogeneous catalyst, based in each case onthe mixture guided to the first membrane module, in the resultingpermeate stream from the first membrane module and enriches it in theresulting retentate stream from the first membrane module,

the retentate stream from the first membrane module is guided to thesecond membrane module, which depletes the homogeneous catalyst, basedin each case on the retentate from the first membrane module, in theresulting permeate stream from the second membrane module and enrichesit in the resulting retentate stream from the second membrane module,

characterized in that

the at least two membrane modules are joined sequentially to one anotheron the permeate side in such a way that the at least two permeatestreams form an overall permeate stream in which complete mixing of theindividual permeate streams does not take place, and

a portion of the overall permeate stream, the recycled permeate, isrecycled to the feed vessel and/or beyond the feed vessel but upstreamof the conveying device, and the remainder of the overall permeatestream, the removed permeate, is removed from the membrane stage, withthe recycled permeate having a higher concentration of the homogeneouscatalyst than the removed permeate.

The reaction mixture comes from a reaction zone suitable for therespective method, preferably one or more suitable reactors. Theretentate stream containing at least a majority of the homogeneouscatalyst is preferably recycled to the reaction zone, especially thereactor(s), optionally after prior purification and/or workup of thecatalyst. Since the mass flow rate from the reaction zone can vary forproduction-related reasons, the above-described control- andplant-related functions can also be employed in the separating-off ofhomogeneous catalysts.

In the reaction zone, preferably the reactor(s), a homogeneouslycatalyzed reaction is accordingly conducted. These reactions may be thefollowing: oxidations, epoxidations, hydroformylations, hydroaminations,hydroaminomethylations, hydrocyanations, hydrocarboxylations,hydrocarbonylations, hydrocarboxyalkylations, alkoxycarbonylations,aminations, ammoxidation, oximations, hydrosilylations, ethoxylations,propoxylations, carbonylations, telomerizations, metatheses, Suzukicouplings and hydrogenations.

Preference is given to a hydroformylation. The hydroformylation isespecially a hydroformylation of olefins having 3 to 15 carbon atoms,preferably 8 to 12 carbon atoms. The hydroformylation is preferably ahomogeneously catalyzed hydroformylation in which the catalyst system is(fully) dissolved in the liquid phase of the reaction mixture. Thecatalyst system for the hydroformylation preferably comprises atransition metal from group 8 or 9 of the Periodic Table of the Elements(PTE) and at least one organic phosphorus-containing ligand. Suitablephosphorus-containing ligands are known to the person skilled in theart, but are preferably monodentate phosphorus-containing ligands, forexample tris(2,4-di-tert-butylphenyl)phosphite.

Transition metals used may especially be iron, ruthenium, iridium,cobalt or rhodium, preferably cobalt or rhodium, more preferablyrhodium. Catalytically active species discussed are typically(ligand)-carbonyl complexes of the metal atoms that form in the liquidreaction mixture under elevated pressure and elevated temperature.

The hydroformylation may be conducted in the presence of a solvent, inwhich case the solvent should be compatible with the hydroformylationmethod. Solvents used may be suitable solvents known to the personskilled in the art for hydroformylation, for example alkanes, aromatichydrocarbons, water, ethers, esters, ketones, alcohols and the reactionproducts or by-products of the hydroformylation, such as aldehydes andcondensation products of the aldehydes.

In addition, the hydroformylation can be conducted at a pressure of 10to 400 bar, preferably 15 to 270 bar. The temperature in thehydroformylation may be 70 to 250° C., preferably 100 to 200° C., morepreferably 120 to 160° C.

The present invention is described by the figures which follow, in whichparticular embodiments are shown. The figures constitute merely anillustration and should not be regarded as limiting.

FIG. 1 shows an illustrative construction of a membrane unit comprisinga membrane stage with two membrane modules. The membrane stage consistsof a feed vessel (B-1), a pump (P-1) and two membrane modules (M-2/M-3).The feed vessel (B-1) is fed with the liquid mixture as feed to themembrane stage (F-1). From the feed vessel (B-1), the liquid mixture isguided by the pump (P-1) as feed (F-5) to the first membrane module(M-2), for example a membrane loop, where the first membrane separationin this case takes place. From the membrane module (M-2), the retentatefrom the first membrane module (F-6) and the permeate from the firstmembrane module (F-8) are then withdrawn. The retentate (F-6) is guidedto the second membrane module (M-3), where a further membrane separationtakes place, which gives rise to the permeate (F-9) and the retentate(F-7) from the second or last membrane module (M-3). The retentate (F-7)is withdrawn via the discharge valve (V-2). The two permeates (F-8/F-9)from the two membrane modules (M-2/M-3) are guided to a common pipeline(F-10). The two permeates (F-8/F-9), in the context of the presentinvention, give rise to the overall permeate (dotted rectangle), whichexists at least as calculation variable even without complete mixing.Two permeate streams are withdrawn from the common pipeline (F-10): thepermeate recycled (F-11) and the permeate removed (F-12). The permeateremoved (F-12) is removed via the discharge valve (V-4), and therecycled permeate (F-11) via the return valve (V-3) to the feed vessel(B-1). It would also be conceivable that the permeate recycled (F-11) isfed not into the feed vessel (B-1) but between feed vessel (B-1) andpump (P-2) (not shown).

FIG. 2 shows an exemplary construction of a membrane unit comprising twomembrane stages. The membrane stages each consist of a feed vessel(B-1/B-2), a pump in each case (P-1/P-2) and one or two membrane modules(M-1/M-2/M-3). The feed vessel (B-1) is fed with the liquid mixture asfeed to the membrane stage (F-1). From the feed vessel (B-1), the liquidmixture is guided by the pump (P-1) as feed (F-2) to the membrane module(M-1), for example a membrane loop, where the membrane separation takesplace. From the membrane module (M-1), the permeate (F-4) from the firstmembrane stage and the retentate (F-3) are withdrawn via the dischargevalve (V-1). The permeate (F-4) from the first membrane module (M-1) isguided to the feed vessel (B-2) for the second membrane stage and thenceguided by means of a pump (P-2) as feed (F-5) to the first membranemodule (M-2) of the second membrane stage, for example a membrane loop,where a membrane separation takes place. From the membrane module (M-2),the retentate from the first membrane module (F-6) and the permeate fromthe first membrane module (F-8) are then withdrawn. The retentate (F-6)is guided to the second or last membrane module (M-3) of the secondmembrane stage, where a further membrane separation takes place, whichgives rise to the permeate (F-9) and the retentate (F-7) from the secondmembrane module (M-3). The retentate (F-7) is subsequently guided viathe discharge valve (V-2) to the feed vessel (B-1) of the first membranestage. The two permeates (F-8/F-9) from the two membrane modules(M-2/M-3) are guided to a common pipeline (F-10). Two permeate streamsare withdrawn from the common pipeline: the permeate recycled (F-11) andthe permeate removed (F-12). The permeate removed (F-12) is removed viathe discharge valve (V-4), and the recycled permeate (F-11) via thereturn valve (V-3) to the feed vessel (B-1). It would also beconceivable that the permeate recycled (F-11) is fed not into the feedvessel (B-1) but between feed vessel (B-1) and pump (P-2) (not shown).

1. A method of continuously separating a component from a liquid mixtureusing a membrane unit which comprises at least one membrane stage and isfed with the mixture as feed, wherein a membrane stage consists at leastof a conveying device, at least two series-connected membrane modules,and a feed vessel upstream of the conveying device, wherein the methodcomprises the following steps in which: the mixture from the feed vesselis guided by means of the conveying device as feed to the first of theat least two membrane modules, which depletes the component to beseparated off, based in each case on the mixture guided to the firstmembrane module, in the resulting permeate stream from the firstmembrane module and enriches it in the resulting retentate stream fromthe first membrane module, the retentate from the first membrane moduleis guided to the second membrane module, which depletes the component tobe separated off, based in each case on the retentate from the firstmembrane module, in the resulting permeate stream from the secondmembrane module and enriches it in the resulting retentate stream fromthe second membrane module, wherein the at least two membrane modulesare joined to one another on the permeate side by such means that the atleast two permeate streams are each guided to a common pipeline, whichgives rise to an overall permeate stream in the pipeline, and in thatthe at least two membrane modules are connected to the common pipelineon the permeate side in opposite directions such that the portion of theoverall permeate stream, the recycled permeate, is recycled at leastpartly to the feed vessel and/or beyond the feed vessel but upstream ofthe conveying device, while the remainder of the overall permeatestream, the removed permeate, is at least partly guided out of themembrane stage, with the recycled permeate having a higher concentrationof the component to be separated off than the removed permeate.
 2. Themethod according to claim 1, wherein the concentration of the componentto be separated off in the recycled permeate is greater than in thecommon pipeline between the connections of the first and last membranemodules, and wherein the concentration of the component to be separatedoff in the common pipeline is greater than in the permeate removed. 3.The method according to claim 1, wherein there are no controllingactuators in the common pipeline between the connections of the firstand last membrane modules to the pipeline.
 4. The method according toclaim 1, wherein the recycling of the recycled permeate to the feedvessel and/or beyond the feed vessel but upstream of the conveyingdevice is not carried out by means of a conveying device but byhydraulic means.
 5. The method according to claim 1, wherein the massflow rate of one stream of the three streams selected from the feed tothe membrane unit, the permeate removed, and the retentate from themembrane unit is defined by a preceding or downstream process step, andone further stream of the three streams is controlled toward a targetvalue.
 6. The method according to claim 1, wherein the mass flow rate ofthe permeate recycled can fluctuate and is established depending on themass flow rate of the permeate removed.
 7. The method according to claim1, wherein both the mass flow rate of the permeate removed and thepermeate pressure are established in each case by means of an adjustableflow resistor.
 8. The method according to claim 1, wherein the mass flowrate of the permeate removed, depending directly or indirectly on thefill level of the feed vessel, is subject to preferably continuousclosed-loop control, a feature of which is that—based on a target valuefixed beforehand for the fill level of the feed vessel—the mass flowrate of the permeate removed increases with rising fill level of thefeed vessel and the mass flow rate of the permeate removed decreaseswith falling fill level of the feed vessel.
 9. The method according toclaim 1, wherein the pressure on the retentate side (retentate pressure)is controlled by means of the conveying device with which the feed isfed to one or more membrane modules, and optionally a further actuator,for example a supply pressure regulator, or by means of a combination ofa manometer and an adjustable flow resistor, especially a valve on theretentate side.
 10. The method according to claim 1, wherein the massflow rate on the retentate side (retentate mass flow rate) is controlledby means of a closed-loop mass flow control system on the retentateside, comprising at least a mass flow meter and an adjustable flowresistor, preferably a valve, or by means of a combination of mass flowmeter and the conveying device.
 11. The method according to claim 1,wherein the component is a homogeneous catalyst which is separated froma reaction mixture.
 12. The method according to claim 1, wherein theconveying device is a pump.
 13. A method of continuously separating ahomogeneous catalyst from a liquid reaction mixture using a membraneunit which comprises at least one membrane stage and is fed with thereaction mixture containing the homogeneous catalyst and coming from areaction zone as feed, wherein a membrane stage consists at least of aconveying device, at least two series-connected membrane modules, and afeed vessel upstream of the conveying device, wherein the methodcomprises the following steps in which: the reaction mixture from thefeed vessel is guided by means of the conveying device as feed to thefirst of the at least two membrane modules, which depletes thehomogeneous catalyst, based in each case on the mixture guided to thefirst membrane module, in the resulting permeate stream from the firstmembrane module and enriches it in the resulting retentate stream fromthe first membrane module, the retentate from the first membrane moduleis guided to the second membrane module, which depletes the homogeneouscatalyst, based in each case on the retentate from the first membranemodule, in the resulting permeate stream from the second membrane moduleand enriches it in the resulting retentate stream from the secondmembrane module, wherein the at least two membrane modules are joinedsequentially to one another on the permeate side in such a way that theat least two permeate streams form an overall permeate stream in whichcomplete mixing of the individual permeate streams does not take place,and a portion of the overall permeate stream, the recycled permeate, isrecycled to the feed vessel and/or beyond the feed vessel but upstreamof the conveying device, and the remainder of the overall permeatestream, the removed permeate, is removed from the membrane stage, withthe recycled permeate having a higher concentration of the homogeneouscatalyst than the removed permeate.
 14. The method according to claim13, wherein the reaction mixture is taken from a reaction zone in whicha homogeneously catalyzed reaction is being conducted.
 15. The methodaccording to claim 14, wherein the homogeneously catalyzed reaction isselected from the group of the following reactions: oxidations,epoxidations, hydroformylations, hydroaminations,hydroaminomethylations, hydrocyanations, hydrocarboxylations,hydrocarbonylations, hydrocarboxyalkylations, alkoxycarbonylations,aminations, ammoxidation, oximations, hydrosilylations, ethoxylations,propoxylations, carbonylations, telomerizations, metatheses, Suzukicouplings and hydrogenations.
 16. The method according to claim 2,wherein there are no controlling actuators in the common pipelinebetween the connections of the first and last membrane modules to thepipeline.
 17. The method according to claim 2, wherein the recycling ofthe recycled permeate to the feed vessel and/or beyond the feed vesselbut upstream of the conveying device is not carried out by means of aconveying device but by hydraulic means.
 18. The method according toclaim 2, wherein the mass flow rate of one stream of the three streamsselected from the feed to the membrane unit, the permeate removed, andthe retentate from the membrane unit is defined by a preceding ordownstream process step, and one further stream of the three streams iscontrolled toward a target value.
 19. The method according to claim 1,wherein the mass flow rate of the permeate recycled can fluctuate and isestablished depending on the mass flow rate of the permeate removed. 20.The method according to claim 1, wherein both the mass flow rate of thepermeate removed and the permeate pressure are established in each caseby means of an adjustable flow resistor.